High temperature superconductors are characterized by their property to carry current without losses when cooled below a temperature specific to the respective high temperature superconductor material, said temperature being termed critical temperature. Due to this unique property high temperature superconductors can be advantageously used in a broad range of applications, for example, in the production of hts-transformers, windings, magnets, current limiters or electrical leads.
On temperature raise the hts-material undergoes a transition to its normal conducting state, said transition being called “quenching”. In its normal conducting state a superconductor material has high ohmic properties. This effect is used in fault current limiters.
The same effect can be achieved if a magnetic field or current applied to a cooled hts-material is enhanced to the respective critical value (critical magnetic field Bc and critical current (Ic), respectively) at which the hts-material also quenches and becomes normal conducting.
These effects or a combination of these effects can be used, for example, for designing a self-controlling fault current limiter based on hts-material. Experiments with high current applied to hts materials, however, have shown that frequently thermo-mechanical problems arise which can lead to destruction of the hts-component.
Hts-materials, usually of ceramic nature, in practical are not perfectly homogenous but show inhomogeneities within the material such as blowholes, blisters and pore, respectively, phases with non-superconducting properties (secondary phases) or small cracks (micro cracks). The geometrical extension of such an inhomogeneity can be from about micrometer to millimeter size.
The regions of such inhomogeneities differ with respect of the superconducting properties such as critical temperature, critical current and critical magnetic field from defect-free regions.
Consequently, in case of current flow through a cooled high temperature superconductor regions with material inhomogeneities can locally change to the normal conducting state. The locally increasing resistance in these regions results in an excessive increase of the current flow in the surrounding superconducting areas of the hts-material. Said local current increase is associated with the generation of heat.
In turn, the heated areas start to quench due to the temperature increase. This process is self-triggering and proceeds avalanching and finally results in crack formation in the hts-material due to thermo-mechanical stress. At the final stage an electric arc can ignite at the cracks (about 10 000 K), which results in destruction of the whole area around the heated region (hot-spot) due to local melting.
The whole process is extra-ordinarily brief and takes place within about sixty msec only.
For avoiding formation of such hot-spots it is known to provide the hts-component with an electrical by-pass termed shunt. Such by-pass can be a layer of an electrical conducting metal such as Ag applied onto the surface of the hts-component. In case of overcurrent, when the hts material or part thereof starts to quench and becomes resistive excessive current is bypassed to the shunt and in the result hot-spot formation is avoided.
However, for example in bulk hts components, such as rods or tubes, for providing an effective protection of the overall hts component from hot-spot formation the bypass must cover the whole surface of the hts component, and surrounds the whole perimeter. Otherwise, in regions not covered by the bypass the risk of hot-spot formation remains.
On the other side, if the whole perimeter is covered by the shunt circular currents can be induced in the shunt material. Such induced currents are undesired since they, in turn, generate a magnetic field and heat which can impair the performance of the hts components and the application, respectively, of which the hts component is part.
In addition to bulk hts components thin-film superconductor components are known.
Typically, thin-film superconductors are wires or tapes composed of a substrate onto which a thin layer of superconductor material is applied. Similar to bulk structures for by-passing excessive current the hts layer can be covered with a shunt.
EP 1 383 178 relates to such thin-film superconductor fault current limiter designed to quench in a controlled way without formation of hot-spots in case of fault event.
Along the length of the tape regions with decreased width are provided, so-called constrictions, which are separated by regions of original width of the tape. By suitable selection of the length and cross-section of the superconductor layer at the constriction on one side and at the regions therebetween simultaneous quench of the constrictions is achieved during the initial period of a fault event thereby avoiding concentration of dissipated power in one region only. Further, by varying the thickness of the by-pass layer of both the constrictions and the regions therebetween resistance can be adjusted to alloy the constrictions to become dissipative already at the initial period whereas the regions therebetween become normal conducting at longer times only. Here, the shunt layer covers the hts layer over its whole width.
Similarly, JP 5022855 suggests to provide a plurality of regions with reduced cross-section along the extension of a superconductor in a regular manner. In case of fault current, these regions with reduced cross-section quench simultaneously already at the initial period of the fault event thereby limiting the excess current. During the course of the fault heat generated in said regions with reduced cross-section is expanded to the region therebetween and promotes uniform quenching of these regions. No shunt is disclosed at all.
Also DE 100 14 197 relates to thin-film superconductor fault current limiters and to the promotion of uniform quenching. Again, over the whole surface of the superconductor layer artificial weak points are distributed. These weak points can be generated by reduction of the layer thickness or by reduction of the critical current density by, for example, doping with impurities. For by-passing excessive current and for promoting expansion of heat generated the whole surface of the tape is covered by a shunt material.
In none of these documents the problems associated with a shunt covering the whole surface of a bulk hts component is addressed.
Objects and Summary:
It was the object of the present invention to avoid uncontrolled hot-spot formation and local burnout of a bulk superconductor component at areas with in homogeneities.
In particular, it was the object of the present invention to provide a bulk hts component suitable for a plurality of applications which is protected against hot-spot formation without the need of covering the whole surface of the hts component with a shunt.
The object of the present invention is solved by a high temperature superconductor component which is provided with at least one region of reduced wall thickness, wherein within that at least one region of reduced wall thickness an electrical shunt is provided.
Said regions of reduced wall thickness, typically, are depressions within the surface of the high temperature superconductor component. The depressions, preferably, have a linear shape extending at least partially over the surface of the hts component.
The present invention is particularly useful for bulk high temperature superconductors of ceramic nature. Such bulk ceramics can, for example, be obtained by compression, for example isostatic compression, or by a melt casting process.
The bulk component can be massive with the cross section through the high temperature superconductor component being entirely filled with high temperature superconductor material. The high temperature superconductor component may, however, also be hollow, that is to say a cross section through the component has a free surface enclosed by high temperature superconductor material. In the scope of the present invention, both massive and hollow high temperature superconductor components may be used, which in a preferred embodiment may be designed as tubes or as rods. Examples for suitable high temperature superconductor components are found, for example, in WO 00/08657, to which reference is expressly made here.
For the present invention any ceramic oxide high temperature superconductor may be used. Preferably, the ceramic oxide high temperature superconductor is selected from the group consisting of bismuth-based, thallium-based, yttrium-based, and mercury-based ceramic oxide superconductors. Typical examples comprise ceramic oxide high temperature superconductors based on Bi-Ae-Cu—O, (Bi, Pb)-Ae-Cu—O, (Y, Re)-Ae-Cu—O or (Tl, Pb)-Ae, Y)—Cu—O or Hg-Ae-Cu—O. In the above formulas Ae means at least one alkaline earth element, particularly, Ba, Ca and/or Sr.
Re means at least one rare earth element, particularly Y or a combination of two or more of the element Y, La, Lu, Sc, Se, Nd or Yw.
Particularly, suitable ceramic oxide high temperature superconductors are those known by the reference BSCCO-2212, BSCCO-2223, wherein the numerical combinations 2212 and 2223 stand for the stoichiometric ratios of the elements Bi, Sr, Ca and Cu, in particular those, wherein part of Bi is substituted by Pb; and those known by the reference YBCO-123 and YBCO-211, wherein the numerical combinations 123 and 211 stand for the stoichiometric ratios of the elements Y, Ba and Cu.